The invention relates to a temperature control system and, more particularly, to a temperature control system which is capable of controlling temperatures within a selected temperature band when a number of heating or cooling sources are used.
A conventional multi-stage heating system initially draws all heating requirements from a single source by switching that source "on" while maintaining the other heating sources in the "off" position. In a two-stage heating system which utilizes a heat pump and an electrical resistance heater, the initial source of heat is usually the heat pump because it is more efficient than its companion electrical resistance heater. When the initial source or heat pump is no longer capable of supplying the heat demanded, the second source is activated to provide additional heating to the system. When the additional stage or heat source is no longer needed, that stage is turned off. A heating system with three or more stages operates in a similar manner. The heating sources are sequentially activated or deactivated depending upon the ambient conditions.
A conventional multi-stage system uses a separate thermostat for each heat source or stage. Each thermostat is set at a different temperature range so that the staged heat sources turn on at successively lower temperatures. A typical mercury thermostat has a temperature sensing range of about 11/2.degree. F., and the thermostats are placed about 1.degree. apart. Accordingly, a two-stage unit would have a control range of 4.degree., without considering drop. A three-stage unit would have a control range of about 61/2. These relatively wide temperature ranges can cause inefficient energy use and user discomfort. In addition, wide temperature variations may necessitate frequent manual resetting of the temperature control.
Recent efforts to achieve increased energy efficiency have rekindled interest in the proportional control of heating and cooling systems. A multi-stage thermostat is used in proportional control systems and is a limiting factor in the number of stages employed.